The present invention relates to a preparation method for image information by an image processing device.
A PET device has been used, as an important tool, mainly for the object of diagnosis of malignant tumors in the medical field, in recent years. The PET device measures radiation (γ-ray) emitted from the inside of a patient body, derived from a radiopharmaceutical injected into a patient as a subject body, and images distribution of the radiopharmaceutical in the patient body from measurement data thereof. Such a PET device is used in diagnosis of metabolic function and physiologic function. As a typical radiation image diagnosis device, which applies radiation measurement technology, for noninvasively obtaining an image of the inside of the patient, there is an X-ray CT device.
The PET device is equipped with many radiation detectors, and is required to process a huge amount of radiation detection signals (γ-ray detection signals), which are output from these radiation detectors. The quality of obtained image information has been limited from restriction of data processing, because image information (PET image information) is reconstructed by processing of the huge amount of data obtained by these radiation detection signals. However, with the rapid progress of signal processing circuit and computer technology, in recent years, image information with high quality has been enabled to be provided.
Diagnosis of a malignant tumor using the PET device is executed as follows. Firstly, the radiopharmaceutical, labeled with a positron emission nuclei species (15O, 13N, 11C, 18F and the like), which specifically accumulates at a specific moiety in a body (hereafter called a PET pharmaceutical), is administered to the patient. A positron, which is emitted from the PET pharmaceutical in the patient body, experiences positron annihilation by binding with an electron inside of a nearby cell. In this annihilation, a pair of γ-rays (hereafter called pair γ-rays) having an energy of 511 keV are emitted. Because each of the pair of γ-rays is emitted in a nearly opposite direction to each other, it can be specified by coincidence counting of both γ-rays, at what position inside the body the positron annihilation event occurred. These γ-rays are detected with the radiation detectors. After the detection of a statistically sufficient number of the pair γ-rays, occurrence frequency distribution of the pair γ-rays, that is distribution of the PET pharmaceutical inside the patient body, can be imaged, by using an image information reconstruction algorithm, such as a filtered-back-projection method (refer to IEEE Transactions on Nuclear Science, Vol. NS-21, pages 228 to 229). Measurement of the γ-rays to be generated, caused by the PET pharmaceutical inside the body, is called emission measurement (hereafter called E measurement), and image information reconstructed, based on γ-ray detection signals obtained by the emission measurement, is called emission image information (hereafter called E image information). The E image information is generally called simply PET image, however, in the present description, to differentiate from transmission image information (hereafter called T image information) to be described later, it is called E image information. In addition, a series of processes from E measurement to reconstruction is called emission imaging collectively.
In an inspection using such a PET device, for example, in the case where the patient is administered with a PET pharmaceutical called FDG (Fluoro-2-deoxyglucose), which is an analog of a saccharide (glucose), the PET pharmaceutical is accumulated at a malignant tumor having a larger saccharide metabolism as compared with a normal moiety. Therefore, diagnosis of position and shape of malignant tumor becomes possible.
Incidentally, in an inspection using the PET device (PET inspection), which requires quantitativeness, in addition to E measurement, there is also executed a measurement called transmission (the transmission measurement, hereafter called T measurement), by using a γ-ray source, which is a transmission radiation source installed at the PET device. Attenuation of a γ-ray in the PET measurement indicates a phenomenon where a γ-ray derived from the radiopharmaceutical is not detected as coincidence count data effective in imaging, as a result of impact of interaction thereof with substances inside a body, before emission outside a patient body. A process to compensate this attenuated amount of this γ-ray is called attenuation compensation, and at present, it is executed in most of the PET inspections.
The attenuation compensation usually uses data obtained by T measurement. That is, by rotating a γ-ray source around the patient lying on a bed, γ-ray radiation emitted from the γ-ray source is irradiated onto the patient. Each of radiation transmittances in various directions, which this γ-ray radiation transmits through the patient, is determined. Data obtained by E measurement is compensated by using data of these radiation transmittances. As the γ-ray source, a radioisotope (hereafter called “RI”) such as 68Ge-68Ga and 137Cs is usually used. Instead of the γ-ray source, an X-ray source, described in JP-A-2006-231083, may also be used. It should be noted that, if necessary, tomographic image information of the patient is reconstructed, based on data obtained by T measurement. This tomographic image information is a morphological image, and is called hereafter T image information. The T image information is one representing distribution of radiation attenuation inside a patient body. If necessary, attenuation rates by each projection direction may be determined again based on the image information, and be used for attenuation compensation of these attenuation rates.
In recent years, a combined PET/CT device has been proposed, where an X-ray CT device is arranged in parallel to and combined with a PET device. This combined PET/CT device executes attenuation compensation by utilization of tomographic image information obtained by the X-ray CT device. JP-A-2006-231083 has proposed a PET device with a structure rotating an X-ray source inside a plurality of radiation detectors arranged circularly. This PET device also executes attenuation compensation by utilization of tomographic image information reconstructed by using detection signals of X-rays emitted from the X-ray source and transmitted through the patient.
As a large factor decreasing quality of E image information, impact of movement of a patient (hereafter called patient motion) is included. In the patient motion, there is periodic movement accompanied with involuntary respiration and heart beat, and a voluntary posture change. Because the PET inspection requires a long measurement time, usually from several minutes to several tens of minutes, it is difficult to suppress patient motion, without giving stress to the patient. In particular, because motion by respiration (hereafter called respiration motion) reaches as large as 2 to 3 cm, even in resting respiration, impact of patient motion on E image information is large in the PET inspection of a moiety near a lung field.
As a method for compensating blurring of image information accompanied with periodical patient motion such as respiration, a method called gated-acquisition has been known. The gated-acquisition is a method for dividing data of E measurement measured over a plurality of patient motion period amounts for each data of patient motion phases, and reconstructing E image information by each patient motion phase, respectively, by using these divided data. For example, in gated-acquisition for respiratory motion, as described in The Journal of Nuclear Medicine, Vol. 45, No. 2, pages 214 to 219, minute change of breath temperature is captured, and as described in The Journal of Nuclear Medicine, Vol. 43, No. 7, pages 876 to 881, movement of the body surface of a chest part is traced with an infrared stereo camera, or the like, and thus respiration phase information is obtained, and data obtained by E measurement, based on this phase information, is divided by each of the patient motion phases.
As another method for compensation of respiratory motion, there is a method for using the combined PET/CT device (refer to The Journal of Nuclear Medicine, Vol. 45, No. 8, pages 1287 to 1292). In The Journal of Nuclear Medicine, Vol. 45, No. 8, pages 1287 to 1292, attenuation compensation is executed by X-ray CT imaging in a cinema-mode, in the complex PET/CT device to obtain E image information by each respiration phase, and by using an X-ray CT image at a corresponding respiration phase, in preparing phase image information of each E image information by the gated-acquisition. It should be noted that, in IEEE Transactions on Medical Imaging, Vol. 18, No. 8, pages 712 to 720, a non-linear superimposing technology for two image informations (Non-rigid image registration method) has been disclosed.